Add a Second Menu to a theme that only support 1 menu

Adding custom menu position to your wordpress theme is fairly simple.
This is usually done this way:

1. Announcing the desired menu positions (in function.php):

add_action( 'init', 'register_my_menus' );

function register_my_menus() {
    register_nav_menus(
        array(
            'first-menu' => __( 'First Menu' ),
            'second-menu' => __( 'Second Menu' ),
            'third-menu' => __( 'Third Menu' )
        )
    );
}

.

2. Embedding them in desired place in your theme:

<?php wp_nav_menu( array( 'theme_location' => 'first-menu' ) ); ?>

.

In Your case & since you already have one custom menu position you first need to find in your functions.php where the current menu was defined and add another one as shown in the above example.

Hope this helps,
sagive.

EDITED (REPLACE THE EXISTING CODE IN YOUR MENU CODE):

function infinity_register_menus() {

    /** Get theme-supported menus. */
    $menus = get_theme_support( 'infinity-core-menus' );

    /** If there is no array of menus IDs, return. */
    if ( !is_array( $menus[0] ) ) {
        return;
    }

    /* Register the 'primary' menu. */
    if ( in_array( 'infinity-primary-menu', $menus[0] ) ) {
        register_nav_menu( 
            array(
                'infinity-primary-menu' => __( 'Infinity Primary Menu', 'infinity' ),
                'secondary-menu' => __( 'Secondary Primary Menu', 'infinity' )
            )
        );
    }
}